# A Smolin: Realistic and anti-realistic interpretations of QM

#### charters

Why should my statement above imply such obvious nonsense of non-existence of free electrons?
Because you reject all possible affirmative claims about where in spacetime the free electron is present. You don't accept it is unsharply, ontologically everywhere that it's quantum state has non-zero amplitude (or amplitude in excess of |0>). You don't accept the Bohmian idea that it is sharply at a single (epistemically uncertain) point. You claim quantum theory is just a theory of detector clicks.

Therefore, you don't believe free electrons are present in spacetime, but somehow pop into spacetime when forming macro objects, and you want to start using chemistry, materials science etc.

#### PeterDonis

Mentor
you reject all possible affirmative claims about where in spacetime the free electron is present
No, he just rejected the two you describe:

You don't accept it is unsharply, ontologically everywhere that it's quantum state has non-zero amplitude (or amplitude in excess of |0>). You don't accept the Bohmian idea that it is sharply at a single (epistemically uncertain) point.
But these by no means exhaust the possibilities. As I understand @vanhees71's position, it is that "electron" is a name for a particular class of states of a particular quantum field (the charged lepton field in the Standard Model). Your description of "unsharply, ontologically everywhere that its quantum state has nonzero amplitude" is a non-relativistic description, so is not a valid description of a quantum field state; and Bohmian mechanics also is a non-relativistic model, so it has the same issue.

#### charters

No, he just rejected the two you describe:
But these by no means exhaust the possibilities. As I understand @vanhees71's position, it is that "electron" is a name for a particular class of states of a particular quantum field (the charged lepton field in the Standard Model). Your description of "unsharply, ontologically everywhere that its quantum state has nonzero amplitude" is a non-relativistic description, so is not a valid description of a quantum field state; and Bohmian mechanics also is a non-relativistic model, so it has the same issue.
The particles identified with the particle number eigenstates of free (or asymptotic interacting) QFT is very much an example of unsharp, ontologically extended entities. See the discussion here: https://arxiv.org/abs/quant-ph/0112149

Indeed, it is the impossibility of strict particle localization or even sub-Compton effective localization in QFT (due to Reeh-Schlieder) which partially motivates the picture of unsharp entities.

The options I offered are logically exhaustive. It is a tautology an entity must be either sharply somewhere, unsharply somewhere/everywhere, or nowhere. Each leads to a different aspect of the measurement problem, and I am just trying to walk vanhees to the version most salient to him.

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#### vanhees71

Gold Member
Because you reject all possible affirmative claims about where in spacetime the free electron is present. You don't accept it is unsharply, ontologically everywhere that it's quantum state has non-zero amplitude (or amplitude in excess of |0>). You don't accept the Bohmian idea that it is sharply at a single (epistemically uncertain) point. You claim quantum theory is just a theory of detector clicks.

Therefore, you don't believe free electrons are present in spacetime, but somehow pop into spacetime when forming macro objects, and you want to start using chemistry, materials science etc.
Well, I like Bohmian mechanics as an alternative interpretation for non-relativistic quantum mechanics. What's still missing is a convincing Bohmian interpretation of relativistic QFT.

Of course, I believe free electrons are present at spacetime. They are well observed and in fact they were the first discovered free elementary particles (Wichert, Thomson 1897). They are described in the Standard model as spin-1/2 Dirac quantum fields. Since they carry no color charge they have asymptotic free states and thus are observable has free particles.

It doesn't make sense to think about them as classical point particles, because this contradicts a plethora of known empirical facts. Despite this, classical relativistic interacting point particles are a mathematical nuissance rather than a simplified description as in non-relativistic physics, but that's another story.

#### charters

Of course, I believe free electrons are present at spacetime
Then you have to answer my question of *where* in spacetime you think they are, between measurements. Everywhere/exactly where the quantum state says or somewhere more sharply defined? I have often found when people see the measurement problem as a non-issue, it is because they aren't asking all the relevant questions, such as this one, to fully vet the logical coherence of their views.

Well, I like Bohmian mechanics as an alternative interpretation
But your whole argument has been that the measurement problem is a non-issue, while the purpose of Bohmian mechanics is to try to deal with the measurement problem. What's there for you to like about it?

#### vanhees71

Gold Member
To the first point: An electron is described as a Dirac field. A free electron is described as a one-particle Fock state, and it's somewhere. Given the state (i.e., the statistical operator) it is prepared in, there's a probability distribution of its location, no more no less. It doesn't take a determined position, but there's only a probability distribution where it'll be found. I don't see a problem in this, because that's how electrons behave in the lab with high precision.

To the second point: It's funny, how you cut my previous comment. I said, Bohmian mechanics is a nice alternative interpretation of non-relativistic quantum mechanis, but it's incomplete, because there's no convincing non-relativistic version of it and thus is incomplete compared to standard QT.

#### charters

It doesn't take a determined position, but there's only a probability distribution
This is a contradiction. A "probability distribution" means the electron has a determined but unknown location. So, I have to ask again, between measurements, do you claim the position is A) determined but unknown or B) objectively undetermined, such that the system is "smeared" across all possible positions?

Alternatively, I would say failing to recognize the significance of this nuance is why you resist the validity of the measurement problem.

To the second point: It's funny, how you cut my previous comment. I said, Bohmian mechanics is a nice alternative interpretation of non-relativistic quantum mechanis, but it's incomplete, because there's no convincing non-relativistic version of it and thus is incomplete compared to standard QT.
But why do you think anything at all is nice about BM/whatsoever care about its completeness, given you deny the measurement problem, which is all BM is about?

#### PeterDonis

Mentor
A "probability distribution" means the electron has a determined but unknown location.
Not if the probability distribution is over measurement outcomes. Such a probability distribution makes no ontological claim at all about the state of the system prior to measurement.

#### charters

Not if the probability distribution is over measurement outcomes. Such a probability distribution makes no ontological claim at all about the state of the system prior to measurement.
Correct. This was explicitly anticipated above. It is also where I thought we were heading until vanhees affirmed the ontological claim that free electrons exist in spacetime, taking this option off the table.

#### PeterDonis

Mentor
It is also where I thought we were heading until vanhees affirmed the ontological claim that free electrons exist in spacetime, taking this option off the table.
I don't see why. Saying that there is a probability distribution over measurement outcomes is perfectly consistent with saying free electrons exist in spacetime but don't have a determined position. The first statement, as you agree, makes no ontological claim at all about the state prior to measurement; the second makes an ontological claim about the state prior to measurement that rules out having a determined position, but since the first statement makes no ontological claim, it does not require the electron to have a determined position.

#### charters

I don't see why. Saying that there is a probability distribution over measurement outcomes is perfectly consistent with saying free electrons exist in spacetime but don't have a determined position. The first statement, as you agree, makes no ontological claim at all about the state prior to measurement; the second makes an ontological claim about the state prior to measurement that rules out having a determined position, but since the first statement makes no ontological claim, it does not require the electron to have a determined position.
But this isn't where the discussion between vanhees and I is currently focused. In #55 I was careful to ask about the nature of the free electron "between measurements." In #56, vanhees said "there's a probability distribution of its location" not a probability distribution of measurement outcomes. So, I worry this is going to un-focus a discussion in which we were already kind of struggling.

#### vanhees71

Gold Member
This is a contradiction. A "probability distribution" means the electron has a determined but unknown location. So, I have to ask again, between measurements, do you claim the position is A) determined but unknown or B) objectively undetermined, such that the system is "smeared" across all possible positions?

Alternatively, I would say failing to recognize the significance of this nuance is why you resist the validity of the measurement problem.

But why do you think anything at all is nice about BM/whatsoever care about its completeness, given you deny the measurement problem, which is all BM is about?
No, it's no contradiction. According to QT the electron doesn't take a determined position, no more no less. The state provides a probability distribution to find an electron at a given position. Where should there be a contradiction? According to QT neither A) nor B) is correct. According to B) the position is objectively determined, but whenever I register an electron it's registered as one electron not some smeared entity.

An interpretation like Bohmian mechanics which doesn't cover relativistic physics is incomplete in comparison to minimally interpreted relativistic QFT. So why should I bother about Bohm's interpretation, which doesn't provide anything except a funny deterministic interpretation of non-relativistic QM but doesn't provide anything more than minimally interpreted QT as far as the physics is concerned and is less complete than the minimally interpreted QT?

#### Auto-Didact

According to QT neither A) nor B) is correct. According to B) the position is objectively determined, but whenever I register an electron it's registered as one electron not some smeared entity.
This makes your claim about states in between measurements to either be 1) that such states are purely epistemic, or 2) you outright reject the use of standard logic w.r.t. physics i.e. you are claiming that discussing Nature explicitly requires a more exotic form of logic.

Option 1 is explicitly ruled out by the PBR theorem. Incidentally, Smolin discusses this in the book.

Option 2 has been tried before in a specific implementation known as quantum logic and the consensus is that this form of logic fails, but the issue is still somewhat open. For years I myself believed the need for an exotic logic to be the answer, but I have long since changed my mind; important to note is that such an extension to standard logic seems to be both unwarranted and unnecessary given that there actually are other interpretations which work perfectly well using standard logic and Occam's razor.

#### Tghu Verd

First I would recommend the book to anyone who reads or takes part in discussions on QM foundations.
Is it comprehensible for a non-QM expert, do you feel, @Auto-Didact?

#### Auto-Didact

Is it comprehensible for a non-QM expert, do you feel, @Auto-Didact?
Yes, I would argue it is. As Smolin makes clear in his talk linked in the first post, he is directly speaking to the public. The book is also written in a style that anyone should be able to read and understand. If you can follow his argument in the talk you should be able to follow his arguments in the book; a minor caveat is that fully understanding a few of the last chapters requires having read his previous books.

#### Tghu Verd

Thanks, I'll give it a go

#### vanhees71

Gold Member
This makes your claim about states in between measurements to either be 1) that such states are purely epistemic, or 2) you outright reject the use of standard logic w.r.t. physics i.e. you are claiming that discussing Nature explicitly requires a more exotic form of logic.

Option 1 is explicitly ruled out by the PBR theorem. Incidentally, Smolin discusses this in the book.

Option 2 has been tried before in a specific implementation known as quantum logic and the consensus is that this form of logic fails, but the issue is still somewhat open. For years I myself believed the need for an exotic logic to be the answer, but I have long since changed my mind; important to note is that such an extension to standard logic seems to be both unwarranted and unnecessary given that there actually are other interpretations which work perfectly well using standard logic and Occam's razor.
Can you explain, why a purely epistemic interpretation is ruled out by the PBR theorem? PBR assume that there is an "ontic state" beyond the quantum mechanical state. Within QT there is no such thing, but only the quantum state, and its meaning is the probabilistic one given by Born's rule. Whether or not there is something beyond the quantum state or not, is not addressed by quantum theory. If you assume that there's an ontic state, the PBR theorem shows that then the epistemic interpretation of the quantum state leads to contradictions, but I don't assume any such thing as an "ontic state". There's not the slightest hint of something like this in any observation of nature, i.e., I don't see why I need the $\lambda$ of the PBR paper (Nat. Phys. 8, 475 (2012)).

#### charters

No, it's no contradiction. According to QT the electron doesn't take a determined position, no more no less. The state provides a probability distribution to find an electron at a given position. Where should there be a contradiction? According to QT neither A) nor B) is correct. According to B) the position is objectively determined, but whenever I register an electron it's registered as one electron not some smeared entity.
I specifically asked about *where* in spacetime you claim the free electron is *between measurements* to avoid this dodge. Saying you believe the ontological premise that the free electron exists in spacetime requires that you commit to a belief about this, independent of any talk about measurements or "finding." I am trying to show that your view is untenable under scrutiny, but you resist the application of this scrutiny by not directly answering plain questions that would lock you to a view and the consequences of it.

So why should I bother about Bohm's interpretation,
That's what I'm asking you. You said you like Bohm, not me.

But ok, I think I'm going to bow out of this discussion, not because you've shown the measurement problem to be trivial, but because there isn't enough recollection of our progress from day to day, so this is going in circles. It will probably be more effective for you to discuss this face to face, I think.

#### vanhees71

Gold Member
I don't claim "the electron is between measurements". It doesn't make sense at all. An electron is prepared somewhere in space. It's position is always indetermined due to the Heisenberg uncertainty relation. Thus its about its position there's always only a probability distribution to find it at a given position, no more no less. It doesn't make sense to talk about an electron or any other entity in physics, including macroscopic bodies without talking about their observability.

A measurement is also nothing than the interaction of the electron with other entities, following the fundamental laws of (quantum) physics. Thereby an electron may even be annihilated (e.g., if you let it collide with a positron and in the collision it's annihilated together with the positron into two photons). Thus you can only say an electron has been prepared in some state at time $t$. About its fate, i.e., whether it will still "be somewhere" (necessarily with a more or less uncertain position) or not, I cannot say anything, if I don't know the complete setup. As the example with the positron shows, it can even be annihilated. Then there's no electron left at all. That's all well-described by quantum theory, without any contradictions (neither intrinsic contradictions nor contradictions with any observation, so far).

#### Auto-Didact

Can you explain, why a purely epistemic interpretation is ruled out by the PBR theorem? PBR assume that there is an "ontic state" beyond the quantum mechanical state. Within QT there is no such thing, but only the quantum state, and its meaning is the probabilistic one given by Born's rule. Whether or not there is something beyond the quantum state or not, is not addressed by quantum theory. If you assume that there's an ontic state, the PBR theorem shows that then the epistemic interpretation of the quantum state leads to contradictions, but I don't assume any such thing as an "ontic state". There's not the slightest hint of something like this in any observation of nature, i.e., I don't see why I need the $\lambda$ of the PBR paper (Nat. Phys. 8, 475 (2012)).
This post demonstrates a clear misunderstanding of what it means to have an ontology: having an ontology means having an actual existence and being ontic simply means actually existing.

Example: Unicorns (one-horned horses) don't have an ontology (or aren't ontic) in the science of biology.

Similarly, any state that actually exists in any literal sense is by definition an ontic state.

If you don't accept this explanation, you are implicitly committing to option 2 from post #63.

#### PeterDonis

Mentor
In #56, vanhees said "there's a probability distribution of its location" not a probability distribution of measurement outcomes.
As his later posts make clear, by "probability distribution of its location" he meant "probability distribution of measurement outcomes when you measure its location". You need to take people's posts in context instead of fixating on a particular phrase.

I specifically asked about *where* in spacetime you claim the free electron is *between measurements* to avoid this dodge.
It's not a dodge, it's a refusal to accept your claim that your categories are logically exhaustive. Your categories are interpretation dependent.

#### samalkhaiat

According to QT the electron doesn't take a determined position, no more no less.
Exactly. And the uncertainty principle is the reason why probability enters QM, full stop. Why do need to participate in this kind discussions? Let me just tell you that if this Smolin guy gave that talk in London or Oxford/Cambridge, he would be, after 15 minutes, talking to an empty lecture theatre in Oxford/Cambridge or get booed in London.

#### charters

As his later posts make clear, by "probability distribution of its location" he meant "probability distribution of measurement outcomes when you measure its location". You need to take people's posts in context instead of fixating on a particular phrase.

It's not a dodge, it's a refusal to accept your claim that your categories are logically exhaustive. Your categories are interpretation dependent.
Then he should have simply continued to agree to this instrumentalist position we were focusing on in the #40s, instead of rejecting it in the #50s (this is the more complete context). The overall categories I gave are exhaustive, and I accept what you suggest here is a logically possible option - in these later posts I am merely assuming some winnowing has taken place. The problem we've had is as soon as I try to get a firm, particular commitment to move us along, they backtrack on the commitment in order to reject the implications of the commitment that would lead to the measurement problem.

I think this is just not an effective medium/format for the socratic approach I was trying, with the strictly linear comment thread.

#### PeterDonis

Mentor
Then he should have simply continued to agree to this instrumentalist position we were focusing on in the #40s, instead of rejecting it in the #50s (this is the more complete context).
The term "instrumentalist" is also interpretation dependent (as are Smolin's terms "realist" and "anti-realist"). This kind of labeling game does not strike me as a good way to make progress.

The overall categories I gave are exhaustive
I don't think they are, and apparently @vanhees71 doesn't think they are either. The Aristotelian game of stating a set of categories, claiming they are logically exhaustive, and then trying to force every claim into one of them also does not strike me as a good way to make progress.

#### charters

I don't think they are, and apparently @vanhees71 doesn't think they are either. The Aristotelian game of stating a set of categories, claiming they are logically exhaustive, and then trying to force every claim into one of them also does not strike me as a good way to make progress.
It is widely agreed any interpretation of QM is a) psi-ontic, b) psi-epistemic, or c) instrumental, and that this is exhaustive. A pseudo-exception are d) dynamical modification a la GRW/Penrose, but this is no longer an interpretation of quantum theory per se, but instead a new (but no less problematic) theory in its own right. See for example Aaronson's classification at the end of this article for an endorsement of the classification scheme: https://www.nature.com/articles/nphys2325?draft=journal&platform=oscar

However, you or vanhees can easily change my mind by presenting a concrete counter-example - just summarily saying this quite standard classification scheme is incomplete is unfair. But so far, this thread has not shown a counter example. It has just been a sort of whack-a-mole game of shifting between the familiar views, though too quickly for me to hone in on the version of the measurement problem most salient to each (which is all I was hoping to do).

It is easy to deny the significance of the measurement problem when one picks and chooses only the good parts of different, incompatible philosophical stances at the necessary times. The hard thing is sticking to a firm commitment, being clear eyed about the weird or difficult implications of it, and then working on a solution.

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